Asian men constitute a large proportion of the population of the world and the U.S., but there have been no prospective studies of their risk of vertebral fractures. In preliminary studies, we found that Asian men have lower areal bone density than do Caucasian men. However, instead of having a higher prevalence, Asian men appear to have a lower prevalence of vertebral fractures. Understanding this paradox will elucidate the causes of vertebral fracture in men--and women. Caucasian men are much larger than Asian men. We hypothesize that the difference in fracture rates, is due primarily to differences in bone size and body size. We propose to add two measurements: follow-up lateral spine radiographs and quantitative computed tomography (QCT) to the Hong Kong arm (consisting of 2,000 men) of the ongoing MrOS U.S. study (consisting of 6,000 men and funded by NIAMS and NIA). This will allow the MrOS study to prospectively test the hypothesis that smaller Asian men have a lower incidence of vertebral fracture than do Caucasian men. We will test the hypothesis that Asian men have higher volumetric BMD (by QCT) and that they have a higher compression strength than the larger Caucasian men. We hypothesize that these differences will be large enough to account for the lower risk of vertebral fractures in Asian men. Finally, by assessing risk factors in the same way in the US and Hong Kong studies, we will be able to test the hypothesis that differences in environmental exposures and other risk factors account for a portion of the differences in vertebral fracture rate. These hypotheses must be tested in cohorts of Caucasian and Asian men that are assessed in the same ways, as we are doing in the MrOS study. This study efficiently builds on the ongoing MrOS study, using the same protocols and systems for data collection and management. Furthermore, the proposed study leverages the fact that Hong Kong Jockey Club Center for Osteoporosis Care and Control will provide the study staff, dual X-Ray densitometry measurements, and measurements of other important covariates. The proposed study presents a unique opportunity to substantially and efficiently add to the understanding of the role of body size, bone size, bone geometry, bone density and environmental factors in the pathogenesis of vertebral fractures.